Make javaslang.collection.Iterator a Traversable

[danieldietrich]

To do this, we (re-)add TraversableOnce. All methods of Traversable are move to TraversableOnce and Traversable extends TraversableOnce.

Then Iterator extends TraversableOnce.

Additionally we add two methods to TraversableOnce:

• boolean hasDefiniteSize() // Iterator, Stream , Gen and Arbitrary return(s) false, all others return true
• boolean isTraversableAgain() // Iterator returns false, all (others) return true

Note: TraversableOnce has undefined size and is not traversable again by definition.

[danieldietrich]

Add zip*/unzip to Iterator. More methods missing? Look at https://github.com/scala/scala/blob/v2.12.0-M2/src/library/scala/collection/Iterator.scala